The present invention relates generally to high frequency signal measuring systems and more particularly to an apparatus and method for measuring the voltage standing wave ratio on a radio frequency transmission line.
Sometimes a portion of the energy of a signal being transmitted along a radio frequency (RF) conductor in an RF transmission circuit is reflected back along the conductor by disturbances in the circuit. These disturbances often occur when an RF conductor, or transmission line, is terminated, such as at a transmitting antenna or at other types of transmission line junctions.
When a reflected signal is present, the voltage on an RF transmission line can be considered to have two components, an incident voltage E.sub.i and a reflected voltage E.sub.r. Similarly, the current can be considered to have two components, the incident current I.sub.i and the reflected current I.sub.r.
The efficiency of an RF transmission system can be determined by comparing the incident voltage E.sub.i with the reflected voltage E.sub.r. A direct comparison of these parameters is the reflection coefficient: p=E.sub.r / E.sub.i. A customary measure of efficiency of RF transmission circuits is the voltage standing wave ratio (VSWR), which is defined by the expression VSWR=(1+p)/(1-p).
When reflected signals are present on a transmission line, the resultant signal may be considered the vector sum of the forward or incident signal and the backward or reflected signal. If the reflected signal has an amplitude of appreciable amplitude relative to the forward signal, the resultant signal can differ considerably from the signal which was applied to the transmission line, i.e., the forward signal. Because the reflected signal can distort the forward signal, it is often desireable to measure the resultant signal so that adjustments may be made to the transmission circuit to reduce or compensate for the disturbances causing the reflected wave.
Various instruments have been devised to measure the resultant signal and/or its component signals. Conventional watt meters, for example, may be used to measure the average power in the resultant signal, if the forward signal has a sufficiently low frequency. It is also known to utilize directional devices which extract a fraction of the power associated with only one of the signals, either incident or reflected, while, ideally, being unaffected by the signal in opposite direction. It is also known to combine two directional devices in a single device so that a measure of net power delivered by the resultant signal to an arbitrary load may be measured.
One form of apparatus for measuring the relative magnitude of the incident and reflected voltages, shown in U.S. Pat. No. 3,683,274, derives a sample voltage proportional to the transmission line voltage and utilizes a current transformer to induce a voltage proportional to the current in the transmission line. The induced voltage is then divided into two equal voltages, one of which is added to the sample voltage while the other is subtracted from the sample voltage.
The sum voltage is proportional to the transmission line's incident voltage and the difference voltage is proportional to the reflected voltage. Through the appropriate conventional circuitry, the sum and difference voltages may be manipulated to derive the reflection coefficient or VSWR. Examples of VSWR measurement instruments which manipulate these types of the sum and difference voltages may be found U.S. Pat. Nos. 3,020,529 and 4,110,685.
Current transformers, which are used to derive the induced voltage, typically have several drawbacks that limit the accuracy of the measurement of the sum and difference voltages. For example, at low frequencies the current transformer may cause a phase shift of unknown magnitude. The current induced in a current transformer leads the induced voltage by a phase of 90.degree., and the inductive reactance of the transformer's secondary winding causes a further phase shift of almost 90.degree.. The difference between this total phase shift and 180.degree., represents an error of generally unknown magnitude.
Another drawback with typical current transformers, one that is accentuated at high frequencies, stems from the transmission line effects of the current transformer's secondary winding. These effects cause a phase delay between the transformer's output and its input, which causes error in a measuring device using a current transformer. These two drawbacks combine to limit the frequency bandwidth over which the measuring device may be used accurately.
The present invention is intended to overcome the above-mentioned disadvantages in the prior art.
It is accordingly an object of this invention to provide a novel method and apparatus for compensating for high frequency phase delay in a circuit for measuring the incident and reflected wave magnitudes on an RF transmission line.
It is another object of this invention to provide a novel method and apparatus for compensating for low frequency phase shift in a circuit for measuring the incident and reflected wave magnitudes on an RF transmission line.
It is still another object of the present invention to provide a novel method and apparatus for increasing the frequency bandwidth over which the incident and reflected wave magnitudes on a RF transmission line may be measured.